The present invention relates to a power controlling circuit for automatic temperature regulator or the like using a zero voltage switching control.
To control the current flowing to a heater for an automatic temperature regulator or the like to control the temperature, there is known phase control circuitry capable of continuously varying the output current and also so-called zero voltage switching control circuitry for controlling said current by turning on or off a switching element such as thyristor, triac or the like at the point where the AC power supply became zero.
However, in power controlling circuits using phase control, the switch element effects its switching operation. Current flow to the load rapidly rises from zero to a value determined by the load every time the switch element is turned on. As the rising current flow generally includes a frequency component corresponding to the AM radio broadcasting band, serious noise interferes with other electronic appliances to be used in connection with the automatic temperature regulator or the like.
In zero voltage switching proportional controls, problems such as the above-described noise interference or the like is removed. In conventional power controlling circuitry, a circuit for detecting the zero voltage point is provided in the power controlling circuit to operate the switching element such as thyristor, triac or the like to turn the elements ON and OFF at the zero-crossover points. Since the state of the zero-cross circuitry is adapted to continue, the energization and deenergization periods become longer than necessary and the control less accurate than the phase control. Furthermore, the number of the switching elements to be used is increased, the cost of the power controlling circuit is higher, the construction is more complicated, and larger. For example, when the control factor is 50% (the energization and deenergization periods are approximately equal) and the resolution is 0.01 (1/100) in 50 Hz one cycle energization. As a result, the energization and non-energization times become longer than desired and the response speed is deteriorated. Also, the problem is serious when the heat source has a small heat inertia, because the resolution is deteriorated when the response is faster.